Photographic elements are widely known to utilize a silver halide grain as an image capture material. The preparation of silver halide grains is also widely known in the art to involve the reaction product of a soluble silver salt with a soluble halide salt in the presence of a peptizer such as gelatin or the like. Subsequent to formation of the silver halide crystals it is desirable to increase the sensitivity of the crystals with spectral sensitizing dyes as known in the art.
During the formation of the desired insoluble silver halide salt a host of deleterious reaction products are typically formed. These deleterious reaction products are predominantly soluble salts which are known to impede the spectral and chemical sensitization and it is therefore desirable to remove these deleterious reaction products.
Methods of removing the deleterious reaction products include a variety of methods which can be categorized as either precipitation or ultrafiltration. Precipitation involves coagulation of the peptizer to form a solid phase, which contains the silver halide grains, and a liquid phase which contains the deleterious reaction products. Removing the liquid phase separates the grains from the deleterious products. Virgin liquid can be added and the peptizer uncoagulated to obtain the desired product. If desired, the coagulation step can be repeated to reach optimal purity. Precipitation methods are undesirable due to the limited control and the poor reproducibility of the impurity levels in the finished product. Furthermore, the addition of a coagulating agent may increase the viscosity of the resulting solution which is undesirable in the subsequent coating of the silver halide grains on a substrate.
Ultrafiltration is an improvement over precipitation. Undesirable soluble reaction products are removed by passing the solution through a filtering means which is chosen to exclude passage of desirable silver halide grains but not soluble reaction products. The exit stream, or flitrate, and retentate can be monitored and the process stopped at a predetermined level of purity which allows for the formation of a product which is more reproducible than those obtained by the precipitation methods.
Ultrafiltration is thought to affect the surface of silver halide grains in a way which is deleterious to the aggregation of some classes of sensitizing dyes. There has been a long felt need in the art to provide a means for supersensitizing ultrafiltered grains such that the advantages of ultrafiltration and the advantages of spectral sensitizing dyes can be realized in combination.